use pdfBox library to render a pdf file as a png.

makePngFromPdf.groovy

import java.awt.image.BufferedImage
import javax.imageio.ImageIO
import groovy.transform.CompileStatic
// pdfbox imports
import org.apache.pdfbox.cos.*
import org.apache.pdfbox.pdmodel.*
import org.apache.pdfbox.pdmodel.common.*
import org.apache.pdfbox.rendering.PDFRenderer

class PdfService {

  // pageNumber is zero indexed for the sake of consistency with PdfBox.
	@CompileStatic
	File makePngFromPdfPage(File doc, int pageNumber = 0) {
		PDDocument document = PDDocument.load(doc)
		// many forms aren't flattened doing so helps ensure they display properly in more pdf renderers.
		try {
			PDAcroForm acroForm = document.getDocumentCatalog().getAcroForm()
			if (acroForm) {
				acroForm.getCOSObject().setItem(COSName.getPDFName('NeedAppearances'), COSBoolean.TRUE)
				acroForm.getCOSObject().setNeedToBeUpdated(true)
				acroForm.refreshAppearances()
				acroForm.flatten()
			}
		} catch(e) {
			log.error('error in flattening form')
		}
		PDFRenderer renderer = new PDFRenderer(document)
		BufferedImage bimg = renderer.renderImageWithDPI(pageNumber, 240) // will be RGB - not black/white or greyscale
		File tempPngFile = File.createTempFile("${doc.key}TempFile", '.png')
		// NOTE: if memory usage or disc IO ends up being a problematic bottleneck consider move to ImageOutputStream instead
		ImageIO.write(bimg, 'png', tempPngFile)
		document.close()
		return tempPngFile
	}
}